Weryk, who is a post-doctoral researcher, was going through data from the Pan-STARRS survey, a project that scans the sky for near-Earth objects.

Weryk is part of a group searching for near-Earth objects, so when he spotted something within the orbit of Mercury, he would typically think it was an asteroid or comet orbiting the sun.

But not this time.

Weryk had seen the object in image archives from the night before. And he knew something was different.

Approaching from above, the object A/2017 U1 was closest to the Sun on Sept. 9. Travelling at 44 kilometres per second, it is headed away from the Earth and sun on its way out of the solar system. (NASA/JPL-Caltech)

“At first it was a ‘that’s weird,’ and then we want to double-check the data,” Weryk told CBC News. “But once we had confirming images from a colleague, we were pretty confident that this was coming from outside the solar system.”

How do astronomers determine whether something is bound in an orbit around the sun or unbound? It’s all about speed.

‘Astronomers have been looking for this for more than a century.’– Peter Brown, Western University

To be bound in an orbit around the sun at Earth’s distance, an object would need to move around 42 kilometres per second. However, when this object was passing through, it was moving at 44 kilometres per second. As well, it appeared that the trajectory of the object was more than enough to resist the gravitational pull of the sun.

After getting other observations to confirm the object’s orbit, Weryk called one of his previous astronomy and physics professors from Western University, Peter Brown, who specializes in meteor physics. Brown was doubtful at first.

“Everybody was quite skeptical about this … but by the time it was announced just a few days ago it was pretty clear from the three dozen observations there was no way to fit a bound orbit to this object,” Brown said.

“Astronomers have been looking for this for more than a century,” he said.

“We expect that, if other planetary systems formed the way ours did, the vast majority of comet-like and asteroid-like bodies don’t get incorpated into the planets, they get scattered out of the system. So there should be tons of these things … all over the galaxy.”

Astronomers have never spotted something like this before, which gave some planetary scientists pause, Brown said.

The problem was that astronomers weren’t looking for that kind of motion for an object, which is different from that of solar system objects. Now that they know what to look for, Weryk and Brown hope that more of these interstellar interlopers will be found.

Brown noted that the object could still be something else. Although it’s highly unlikely, a very large, extremely distant planet could have altered the orbit of something far out in our solar system.

“It’s almost such a long shot, it’s not worth mentioning,” he said.

The object known as A/2017 U1 has only been observed for just over a week, and it will leave our solar system within a few days. So this is the only time astronomers can study it.

If you’re wondering where this object came from, there’s no way of knowing, because during the tens of thousands of years this object has been flying through space, the stars have also been moving (we all orbit the centre of the galaxy).

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